Scintillation detectors have been employed in the oil and gas industry for well logging. These detectors have used thallium activated sodium iodide crystals that are effective in detecting gamma rays. The crystals are enclosed in tubes or casings to form a crystal package. The crystal package has an optical window at one end of the casing which permits radiation induced scintillation light to pass out of the crystal package for measurement by a light sensing device such as a photomultiplier tube coupled to the crystal package. The photomultiplier tube converts the light photons emitted from the crystal into electrical pulses that are shaped and digitized by associated electronics. Pulses that exceed a threshold level are registered as counts that may be transmitted "uphole" to analyzing equipment or stored locally.
The ability to detect gamma rays makes it possible to analyze rock strata surrounding the bore holes, as by measuring the gamma rays coming from naturally occurring radioisotopes in down-hole shales which bound hydrocarbon reservoirs. Today, a common practice is to make measurements while drilling (MWD). For MWD applications, the detector must be capable of withstanding high temperatures and also must have high shock resistance. At the same time, there is a need to maintain performance specifications.
A problem associated with MWD applications is that the detector will report a higher than an actual count rate if the scintillation crystal package produces vibration induced light pulses. The harsh shock and vibration conditions the detectors encounter during drilling can cause a crystal package to emit spurious light pulses in addition to gamma ray induced light pulses. That is, the detector output will be composed of radiation induced counts and vibration induced counts. Heretofore, the detector electronics could not distinguish the vibration induced counts from the genuine gamma counts, whereby the detector reports a higher than actual count rate. The problem is more severe when detecting low level radiation events while the detector is being subjected to a very severe dynamic operational environment.
Some prior art electronic solutions have attempted to filter out vibration induced counts by discriminating on the basis of the pulse shape and/or the signal decay time. These techniques, however, have proven unreliable.